1. Field of the Invention
The present invention relates to a method for efficiently transmitting data over a wireless local area network (LAN), and more particularly, to a method and apparatus for reducing the possibility of collision with other frames and ensuring reliable transmission when transmitting a broadcast/multicast frame in a wireless LAN following a Distributed Coordination Function (DCF) access scheme.
2. Description of the Related Art
As improvement in data transfer rate is of paramount concern in a wireless LAN, a Media Access Control (MAC) sublayer responsible for access control has become increasingly important. However, since both the IEEE 802.11a standard and the IEEE 802.11b standard use a MAC defined in the IEEE 802.11 standard, frequent collisions in the channel access may unavoidably degrade the performance of an overall system in spite of improved data rate. Thus, if occurrence of collisions can be suppressed and errors of a colliding packet can be quickly recovered, the system performance will be significantly improved.
DCF and Point Coordination Function (PCF) are defined in a 802.11 MAC for medium access. The DCF is a basic access mechanism defined in the IEEE 802.11 MAC standard, which provides contention-based medium access services and uses a backoff algorithm for medium access.
In DCF mode, DCF InterFrame Space (DIFS) is a period of time for which a station (STA) must wait to use a medium. If a backoff timer contains a non-zero value after a DIFS period for which the medium remains idle, STAs generate a random backoff time for an additional delay time before transmission. The backoff algorithm is used to minimize collision that occurs when multiple STAs contend for access to the medium for data transmission. An example of the backoff algorithm is given by Equation (1):Backoff Time=Random( )×aSlotTime  (1)where Random( ) is a uniform pseudo random integer, and aSlotTime is one of management information base (MIB) values.
In general, when a STA is operating according to the DCF access mechanism, the STA will transmit a pending MAC Protocol Data Unit (MPDU) if the medium is idle for more than a DIFS period. If carrier sense mechanism determines that the medium is idle under these conditions, a contention window (CW) size is changed by a backoff algorithm. When there is not enough time to transmit or retransmit MPDU and acknowledgement (ACK), the STA may defer this transmission or retransmission by a selected random backoff time.
FIG. 1 illustrates a contention-based access method using DCF. In the IEEE 802.11 DCF mode, Carrier Sense Multi Access/Collision Avoidance (CSMA/CA) is used to access a medium. Of the carrier sense methods, a physical carrier sense mechanism is provided by a physical layer (PHY). For more information, see the 802.11 PHY specifications. A virtual carrier sense mechanism is provided by a MAC layer and uses a Network Allocation Vector (NAV) that is used as a counter at each STA indicating when a channel is idle based on a ‘duration field’ in a frame. A procedure for transmitting a frame according to the CSMA/CA method is as follows: first, a CSMA/CA mechanism determines current status of a medium. If the medium is idle for greater than or equal to a DIFS period, transmission of a frame begins immediately. If the medium is busy, the STA waits until the medium remains idle. If the medium is idle, the STA defers its transmission for DIFS, and if the medium still remains idle for longer than the DIFS period, the STA selects a random backoff time in the range between 0 and a given CW and backs off for the selected backoff time by aSlotTime. If the medium is still idle after the random backoff interval has expired, the STA will begin transmission of the frame.
Four different Interframe Space (IFS) intervals, the time intervals between frames, are defined to provide priority levels for access to wireless media: Short IFS (SIFS), PCF IFS (PIFS), DCF IFS (DIFS), and Extended IFS (EIFS). The relationship between the different IFS intervals is illustrated in FIG. 1. SIFS, the shortest of IFS intervals, is used for an ACK frame, a Clear to Send (CTS) frame, a continuing fragmented frame and a frame sent during a contention free period (CFP). While PIFS is used by only STAs operating under PCF at the start of CFP, DIFS is used by STAs operating under DCF mode to transmit MPDUs and MAC Management Protocol Data Units (MMPDUs). EIFS is used by DCF-based stations when PHY has notified that a frame transmission resulted in a bad reception of the frame due to an incorrect Frame Check Sequence (FCS) value.
FIG. 2 illustrates three conventional unicast frame transmission cases 1-3. Referring to FIG. 2, in case 1 where transmission has been successful, an ACK is received a SIFS interval after transmission of frame 1, and then a backoff algorithm is performed after a DIFS interval before transmission of frame 2.
In case 2 where an ACK has not been received after frame transmission, if ACK is not received within an ACK timeout period after transmission of frame 1, the frame 1 is repeatedly retransmitted after an additional random backoff time, within the maximum retry count, until the transmission is successful, thereby increasing transmission reliability. In the case 2 where the ACK has not been received, a CW value is increased from the original value during the backoff procedure. For example, if CW is 15 during the first backoff, CW increases in the range of 15 to 1023 during the additional backoff after collisions, thereby reducing the probability of collision. This is because, assuming that two STAs exist, the probability of the two STAs colliding during a backoff procedure is 1/CW.
In case 3 where collision has occurred during receipt of an ACK frame after transmission, a backoff procedure is performed after a DIFS or EIFS interval, and frame 1 is retransmitted depending on the status of the ACK frame after an ACK timeout period, thereby reducing the possibility of collision. In the case of unicast frame transmission described above, retransmission of a frame is determined depending on the receipt of ACK, thereby ensuring reliable frame transmission.
FIG. 3 illustrates two conventional broadcast (BC) frame transmission cases 4 and 5. The same is true when a multicast frame is used instead of the BC frame. Referring to FIG. 3, in case 4 where transmission is successful, a backoff procedure is performed after a DIFS interval after BC frame 1 has been transmitted, without receipt of an ACK, and then BC frame 2 is transmitted. In case 5 where transmission has failed due to collision, even when BC frame 1 fails to be transmitted due to collision, a backoff process is performed after the DIFS interval, and then BC frame 2 is transmitted instead of having to retransmit the BC frame 1 as shown in the case 4. Since it is impossible to retransmit frames after the collision, and CW is fixed to a previous value, i.e., Cwmin=15, during the additional backoff procedure after collision, these conventional transmission still have the possibility of collision under the same environment.
As illustrated in FIG. 2, the DCF mode defined in the IEEE 802.11 standard uses a collision avoidance mechanism known as a random backoff to access a medium. Unlike a collision detection mechanism, the collision avoidance mechanism determines whether transmission is successful by the receipt of an ACK corresponding to the transmitted frame. However, as shown in FIG. 3, in the conventional transmission, there is no way to sense or recover from a collision during data transmission due to the absence of an ACK responding to broadcast/multicast data transmission. Thus, the conventional method has a high possibility of collision between different STAs during data transmission, thereby significantly degrading the stability of a wireless network.